LITHOCHYTRIDIDAE Ehrenberg, 1846

Family LITHOCHYTRIDIDAE Ehrenberg, 1846

Lithochytrina Ehrenberg, 1846: 385 [as a family]; 1847: 53 [as a family]. — Schomburgk 1847: 124, 125 [as a family]. — Ehrenberg 1876: 156.

Lychnocanida Haeckel, 1882: 432 [below tribe].

Lithornithida Haeckel, 1882: 436 [nomen dubium, below tribe].

Lychnocaniinae – Petrushevskaya 1971a: 227-228; 1981: 239-240. — Afanasieva et al. 2005: S296. — Afanasieva & Amon 2006: 144.

Lychnocaniidae – Petrushevskaya & Kozlova 1972: 552. — Petrushevskaya 1975: 583; 1981: 229-230; 1986: 133. — Dumitrica 1979: 34. — Kozlova 1999: 127. — Afanasieva et al. 2005: S296. — Afanasieva & Amon 2006: 144. — Suzuki in Matsuzaki et al. 2015: 50.

Lithochytridinae – Petrushevskaya 1981: 244. — Afanasieva et al. 2005: 296. — Afanasieva & Amon 2006: 144.

TYPE GENUS. — Lithochytris Ehrenberg, 1846: 385 [type species by subsequent designation (Campbell 1954: D132): Lithochytris vespertilio Ehrenberg, 1874: 239].

INCLUDED GENERA. — Dictyopodium Ehrenberg, 1847: 54 (= Pterocanarium n. syn.; Pterocanidium and Pleuropodium synonymized by Riedel & Sanfilippo 1970: 529; Lychnodictyum synonymized by Lazarus et al. 1985: 196). — Inversumbella Nigrini & Caulet, 1992: 150. — Lithochytris Ehrenberg, 1846: 385 (= Lithochytridium with the same type species; Sethochytris synonymized by Petrushevskaya 1981: 247). — Lychnocanissa Haeckel, 1887: 1226 (= Acerahedrina, Acerocanium, Lychnocanoma synonymized by Riedel & Sanfilippo 1970: 529; Podocyrtecium n. syn.). — Lychnocanium Ehrenberg, 1846: 385 (= Dictyophimium with the same type species; Lithochytrodes synonymized byPetrushevskaya 1975: 583; Lychnocanella n. syn.). —? Verutotholus O’Connor, 1999: 13.

INVALID NAME. — Tetrahedrina.

NOMINA DUBIA. — Lithornithium, Tetraedrina.

NOMEN NUDUM. — Fenestracanthia.

DIAGNOSIS. — Lithochytrididae consist of two or three segmented Lithochytridoidea with four collar pores on the cephalic basal ring, a free A-rod in the cephalic cavity, and a very short to very long apical horn on the spherical cephalis. Three (rarely two) feet or wings are always present. The cephalic initial spicular system consists of MB, A-, D-, V- and double L-rods. No l-rods are observed. The basal ring is directly connected to the apical side end of MB, V-, and the double L-rods. Furthermore, the basal ring is sharply bended along the line, with the double L-rods. The apical side of the basal ring is merged with the shell wall but all four collar pores are easily recognizable. The V-rod occasionally extends outward from the cephalic wall. The MB is oriented slightly toward to the apical side. The double mp -arch (upper arch of the double AL-arch in the cephalis) is embedded in the cephalic wall and is occasionally visible, near the uppermost A-rod on the cephalis, under light microscopy. The D- and double L-rods merge with the shell wall, forming wall rims. These rods are also connected with each foot.

A protoplasm is observed in Dictyopodium. The endoplasm is transparent and appears as four lobes below the cephalis. The size of the lobe is variable, from very small near the cephalis to large close to the aperture. Many algal symbionts are distributed around the lobes. The endoplasm is observed inside or outside the shell. The pseudopodia are found radiating around the thorax. An axial projection is absent.

STRATIGRAPHIC OCCURRENCE. — Early Paleocene-Living.

REMARKS

The problem in defining Lithochytrididae originates from the poorly reported cephalic structure in the type genus Lithochytris and its closest genus Lychnocanium. The cephalic structure was only reported for Lithochytris (Nishimura 1990: figs 28.2, 28.3) and Lychnocanium (Nishimura 1990: fig.28.1). By contrast, the cephalic structure was repeatedly illustrated in Dictyopodium (Dumitrica 1973a: pl. 13, figs 3-6; Nishimura & Yamauchi 1984: pl. 30, figs 7, 9; Nishimura 1990: fig. 29.7; Sugiyama et al. 1992: pl. 24, figs 5-8, pl. 25, figs 1, 3, 4). To the best of our knowledge, the detailed structure of Dictyopodium is nearly the same as that of Lychnocanium but the former displays the most representative characters of Lithochytrididae. The cephalic structure of Lychnocanissa (originally Lychnocanium) was only observed in the late Eocene to early Miocene specimens (O’Connor 1997a: pl. 9, figs 9-12; 1999; pl. 4, figs 12-15, pl. 4, figs 22-27). However, their cephalic base is completely separate from the shell test while the basal ring is affixed to the shell test by numerous short radial beams. These characters are typical for the Theoperidae. High species diversity is documented in Lychnocanissa (originally Lychnocanoma) (Riedel & Sanfilippo 1970; Petrushevskaya 1981). Nonetheless, several species may not belong to this genus. Verutotholus is tentatively included in the Lithochytrididae though this genus has a double AL-arch, six collar pores and the presence of double l-rods (O’Connor 1999: pl. 2, figs 16, 20, 22b), more closely resembling the Bekomidae. An endemic Lychnocanissa with only two feet was originally described as Acerahedrina (Vinassa de Regny 1900). In regard to the cephalic structure of the Acerahedrina -form of Lychnocanissa (Nishimura 1990: figs 27.4-27.6; Sugiyama & Furutani 1992: pl. 17, fig. 2), the character of the cephalic base is similar to that of Dictyopodium and, thus, this form evidently belongs to the Lithochytrididae. The two feet of the Acerahedrina - form of Lychnocanissa are aligned parallel to the plane that includes MB-, D-, and V-rods. One of the feet appears to be connected with the D-rod while the other is disconnected from any initial rod.

The members of the Lithochytrididae and the valid genera names in the Lithochytrididae were historically misunderstood. As for the family, all Lithochytrididae genera except for “ Lychnocanoma ” and “ Pterocanium ” (the valid names are Lychnocanissa and Dictyopodium in this paper) were not treated in De Wever et al. (2001). De Wever et al. (2001) placed Dictyopodium in the Mesozoic family Ultranaporidae Pessagno 1977a. The current usage of the latter name has been already discussed and resolved by Matsuzaki et al. (2015: 49-50). The valid genus name and correct type species for Lychnocanium (Sanfilippo et al. 1973: 221; Petrushevskaya 1981: 242), “ Lychnocanoma ” (Riedel & Sanfilippo 1970: 529; Petrushevskaya & Kozlova 1972: 553; Petrushevskaya 1981: 241; Nishimura 1990: 132-133; O’Connor 1997a: 77-78; 1999: 24), and “ Pterocanium ” (the valid genus name is Dictyopodium in this paper) (Riedel & Sanfilippo 1970: 529; Petrushevskaya 1981: 237) were not fully agreed among previous researchers. The main argument concerned whether Lychnocanium lucerna Ehrenberg 1847 or Lychnocanium falciferum Ehrenberg, 1854 was the correct type species of Lychnocanium. A similar problem occurred between Pterocanium proserpinae Ehrenberg 1859 and Lithocampe aculeata Ehrenberg 1844b for the correct type species of Pterocanium.

The Lithochytrididae are commonly found from late Eocene sediments to the modern ocean, but the study of their evolution is limited. The evolutionary history of the genus Dictyopodium (= Pterocanium in original) was documented (Lazarus et al. 1985). One solution for the evolution of Lithochytris - Lychnocanium was proposed (Kling 1978: 234; Riedel & Sanfilippo 1981: fig. 12.11).

Knowledge of the living status of the Lithochytrididae is mainly based on Dictyopodium (Matsuoka 1993a: fig. 2.5; 1993b: pl. 5, figs 1, 2; 2017: fig. 23; Suzuki & Aita 2011: fig. 5N; Suzuki & Not 2015: fig. 8.11.17). Algal symbionts of Dictyopodium praetextum (Ehrenberg) were identified as Gymnoxanthella radiolariae but Brandtodinium nucleate remained absent. The identified symbiont is the same dinoflagellate species as those found in Acanthodesmia (Acanthodesmiidae) and Dictyocoryne (Euchitoniidae, Spumellaria) (Yuasa et al. 2016).

VALIDITY OF GENERA

Dictyopodium

The concept of the valid genus Dictyopodium is equivalent to the current usage of Pterocanium because Dictyopodium trilobum, the type species of Dictyopodium, has been classified under the current concept of Pterocanium for more than 130 years (Haeckel 1887). The type species of Pterocanarium is Pterocanium proserpinae, which has been classified in Pterocanium for over a century, but was synonymized with Podocyrtis charybdea by Petrushevskaya (1971a). Lazarus et al. (1985) reconstructed the phylogeny of late Neogene Pterocanium to include Pterocanium charybdeum and Pterocanium trilobum (originally P. charybdeum trilobum); subsequently, Pterocanarium was synonymized with Dictyopodium, and the genera Pterocanidium, Pleuropodium, and Lychnodictyum were synonymized with Dictyopodium. Among these, the oldest available name is Dictyopodium.

Lychnocanissa

In the Atlas, Lychnocanissa corresponds to Lychnocanoma sensu Foreman (1973b: 437), Sanfilippo et al. (1973: 221), Morley & Nigrini (1995: 80), and Suzuki in Matsuzaki et al. (2015: 50) and to Lychnocanium sensu Riedel & Sanfilippo (1970: 529), Petrushevskaya & Kozlova (1972: 553), Petrushevskaya (1981: 242), Nishimura (1990: 132-133), Kozlova (1999: 128), and O’Connor (1997a: 77-78; 1999: 24). Our concept is also equivalent to a combination of Lychnocanium and Lychnocanoma, sensu Tochilina & Vasilenko (2018a: 23). The type-illustration of Podocyrtecium shows three segmentations (Haeckel 1887: pl. 72, fig. 1), but this is likely incorrect because no three-segmented specimens similar to Lychnocanoma have been found. Lychnocanissa, Lychnocanoma, and Podocyrtecium were simultaneously published in Haeckel (1887: 1226 for Lychnocanissa, 1229 for Lychnocanoma and 1339 for Podocyrtecium). Selecting a valid genus is problematic. Campbell (1954: D124) considered Lychnocanissa the nominate subgenus of Lychnocanium. However, the type species of Lychnocanium, Lychnocanium lucerna, was not originally included in Lychnocanissa. Species included at that time included Lychnocanium falciferum, Lychnocanium fenestratum, Lychnocanium fortipes, Lychnocanium sigmopodium, Lychnocanium tetrapodium, Lychnocanium trichopus, and Lychnocanium tuberosum. Campbell (1954) did not designate a type species for Lychnocanissa. Therefore, we newly designated Lychnocanium falciferum as a type species in the Atlas. Lychnocanium falciferum was designated as the type species of Lychnocanium by Campbell (1954: D124), but many authors have commented that this designation is incorrect. One of the authors of the present study (NS) suggested that Lychnocanoma be validated several times over a period of 2 years; however, the final consensus is that Lychnocanissa is a valid genus. The validation of Lychnocanissa over Lychnocanoma obviously violates ICZN (1999) article 23.9, which states, “ 23.9.1. Prevailing usage must be maintained when the following conditions are both met: 23.9.1. The senior synonym [...] has not been used as a valid name after 1899, and 23.9.1.2. The junior synonym [...] has been used for a particular taxon, as its presumed valid name, in at least 25 works, published by at least 10 authors in the immediately preceding 50 years and encompassing a span of not less than 10 years.”

Lychnocanium

There are two concepts of Lychnocanium, one based on the designation of Lychnocanium falciferum as the type species by Campbell (1954: D124), and the other based on the designation of Lychnocanium lucerna, the first species assigned to Lychnocanium, as the type species by Ehrenberg (1847). This confusion has continued since the early 1970s (Riedel & Sanfilippo 1970; Sanfilippo et al. 1973; Petrushevskaya 1981; O’Connor 1997a; Tochilina & Vasilenko 2018a). The only correct solution is for Lychnocanium lucerna to be designated as the type species, according to ICZN (1999) article 69.3, which states, “ Type species by subsequent monotypy. If only one nominal species was first subsequently included in a nominal genus or subgenus without included species, that nominal species is automatically fixed as the type species, by subsequent monotypy. ” Under this monotypy, the concept of Lychnocanium sensu Riedel & Sanfilippo (1970: 529), Petrushevskaya & Kozlova (1972: 553), Petrushevskaya (1981: 241), Nishimura (1990: 132-133), Kozlova (1999: 128) and O’Connor (1997a: 77-78; 1999: 24), and Tochilina & Vasilenko (2018a: 23) is incorrect.

Under the correct type species, Lychnocanium is synonymized with Lithochytrodes and Lychnocanella. Dictyophimium has the same type species as Lychnocanium. Lychnocanella was defined by Kozlova (1999: 127) and is translated as follows: “ Threesegmented shell with three well-developed feet, protruding from the base of the thorax. This genus differs from Lychnocanium Ehrenberg [note: type species L. lucerna in this case] only by a pear-shaped thorax with an elongated proximal part. ” This difference is a major distinguishing feature at the species level but not at the genus level. Petrushevskaya (1981: 241) strongly disagreed with the relationship between Lychnocanium and Lychnocanella, but this opinion was based on the incorrect type species, L. falciferum, not L. lucerna. Lithochytrodes was synonymized with Lychnocanella by Petrushevskaya (1975: 583). Later comments by Petrushevskaya (1981: 256) are translated as follows: “ Characteristics similar to those of Lychnocanella. Differences include a more precise distinction between the second and third segments [...] about 10-20 longitudinal rows of pores [...] Lithochytrodes is proposed as a subjective synonym of Lychnocanella.” Among these groups, Lychnocanium is the oldest available name.